added new argument, --clear_datastore, that tells appscale to erase all zk and cassandra data
added new arg to terminate script to indicate when we want data erased or persisted
not requiring cloud admin to exist if existing data is in datastore (since that user may already be there)
added in mocks to check for user existence before creation
Merge pull request #276 from shatterednirvana/persistence-finally

Persists data across AppScale deployments